1. Field of the Invention
The present invention relates generally to automatic focusing apparatuses, and more specifically, to an automatic focusing apparatus for automatically matching the focus to an object in response to a video signal obtained from an image sensor, in an image sensing apparatus such as a video camera having an automatic focusing function.
2. Description of the Background Art
Conventionally, in an automatic focusing apparatus for use in an image sensing apparatus such as a video camera, an approach utilizing a video signal obtained from an image sensor itself for evaluation of the state of focus control has been developed. Such an approach is essentially free from parallax and possesses excellent characteristics such as, for example, the focus can exactly be matched even if the depth of field is small and an object is located in the distance. Furthermore, according to this approach, a special sensor for automatic focusing does not need to be separately provided, and therefore the apparatus is of a very simple.
As one example of such a focus control method utilizing a video signal, a so-called hill climbing servo system has been conventionally known. An automatic focusing apparatus utilizing the hill-climbing servo system is disclosed, for example, in U.S. Pat. Nos. 4,922,346 and 5,003,339, and briefly stated, a high frequency component of a video signal obtained from an image sensor is detected for every field as a focus evaluating value, the detected focus evaluating value is always compared with a focus evaluating value detected one field before, and the position of a focusing lens continues to be slightly vibrated so that the focus evaluating value always takes the maximum value.
The level of a video signal obtained from the image sensor is generally set below 50% of the dynamic range of the image sensor, so that the video signal level is not saturated in the high luminance portion of an object.
For example, in the case of video cameras for consumer use, generally, a CCD output signal level is so set that a video (luminance) signal having a level of 100IRE is obtained when light enters with a luminance corresponding to 1/3 of the dynamic range of the CCD, i.e. the image sensor (about 1/6 for image sensing apparatuses for business use).
More specifically, when an output of the CCD is subjected to a 10-bit A/D conversion, the range (dynamic range) which the digital output of the CCD can take is from 0 to 1023, and the value taken by a digital video signal having a luminance level of 100IRE in this case is about 340. The average value of the luminance levels of a video signal in an image sensing stage is in the range of approximately 60-70IRE, and therefore the value taken by the digital video signal in this case is in the range of approximately 200-250.
As described above, since the level of an a video (luminance) signal is set low in a conventional image sensing apparatus, a high frequency component level extracted for an automatic focusing operation is also low, thereby lowering the accuracy of a focusing operation.
More specifically stated, especially when an object with a low luminance is being sensed, the contrast of the object is generally small, and therefore a high frequency component level extracted for a hill-climbing control is also low. Accordingly, a detected focus evaluation value is decreased, thereby lowering the accuracy of detection for the maximum value. This makes it difficult to find the in-focus position with a high accuracy, resulting in an erroneous autofocusing operation.
Meanwhile, even when a usual object having a relatively high luminance is sensed, as described above, the range of a digitized luminance signal only corresponds to about 1/3 of the entire dynamic range of the CCD, a high frequency component level extracted for a hill-climbing control does not take a sufficiently large value. Therefore, even when an object with usual brightness is sensed, the accuracy of the focusing operation is not necessarily high, and an erroneous operation can be caused in an automatic focusing operation.